The gas plasma vapor etching process has recently been utilized for performing operations on semiconductor wafers by exposing the plasma to remove portions of material carried by the semiconductor structure. The extension of reactive plasma etching beyond the patterning of silicon, silicon nitride and silicon oxides to include aluminum etching offers several potential advantages in the production of small geometry integrated circuits. Plasma as compared to chemical etching of metal produces better edge definition, less undercutting, considerably less photoresist adhesion sensitivity, and the elimination of so-called "knee breakdown" due to thinning of the photoresist at sharp edges. This thinning where the aluminum goes over and down the side wall of a cut leads to premature resist failure during wet etching, thus permitting removal of the metal at the near edge.
Although plasmas are not clearly understood, it is known that a special form of chemical materials can be made by exposing the compounds to high energy radio frequencies. Under the influence of these radio frequencies, compounds break down and rearrange to form transitory species with life spans so short that they are difficult to identify. Accordingly, unexpected reactions can be effected in a plasma that are difficult or impossible to effect using more conventional techniques.
It has been recognized that the plasma etching of aluminum presents certain inherent problems which are not easily overcome. Such a realization was presented by Poulsen et al in an article entitled PLASMA ETCHING OF ALUMINUM wherein it was stated that aluminum cannot be plasma etched in common chlorine-based etch gases (e.g., Cl.sub.2 or HCl) due to the protective masking of aluminum by the thin aluminum oxide (Al.sub.2 O.sub.3) layer that forms on aluminum surfaces upon exposure to air. Poulsen accomplished his etching by using boron trichloride (BCl.sub.3) and carbon tetrachloride (CCl.sub.4) plasmas. In this way, it was found that the aluminum oxide could be removed to allow etching of the exposed aluminum via a reaction with chlorine radicals in the plasma. Poulsen used a single etching process to remove both the protective aluminum oxide and underlying aluminum with one etch gas.
In a second article entitled PLASMA ETCHING OF ALUMINUM by Herndon et al at the M.I.T. Lincoln Laboratory, it was taught that a two-step process could be employed by starting with boron trichloride or carbon tetrachloride followed by a second chemical etch of chlorine. The first plasma etching process removed the aluminum oxide coating while the second chemical etch acted to pattern the underlying aluminum layer.
By practicing the methods of Pousen et al., Herndon et al. and others who have attempted to carry out a plasma etching process of aluminum, certain serious difficulties were encountered. For example, boron trichloride, either taken alone or in an argon, nitrogen or helium carrier gas is extremely corrosive. The plasma etching chamber and electrodes deteriorate extremely rapidly when boron trichloride is used to remove the aluminum oxide coating from the aluminum layer to be etched. Carbon tetrachloride is not as corrosive as boron trichloride but has extremely poor selectivity over silicon dioxide (Si0.sub.2) and photoresist. Aluminum is placed upon a silicon dioxide substrate in order to produce a semiconductor device. The aluminum is then protected by the image-wise application of a photoresist layer. An ideal plasma etching process would remove only the aluminum, and not the photoresist, while leaving the silicon dioxide untouched. As stated, carbon tetrachloride as a sole etchant gas does a very poor job in accomplishing these goals.
The use of chlorine to etch aluminum is known and is shown in both articles referred to above. Practitioners have used chlorine with boron trichloride, but its use with carbon tetrachloride is not universal for it was found that chlorine inhibits the etching of aluminum oxide in a carbon tetrachloride system. Herndon et al. implies that experiments employing chlorine as a second etchant gas have been carried out, but applicant has found that the unrestricted use of chlorine as a second stage in the etching process causes isantropic etching. This means that chlorine does not produce acceptable edge definition for chlorine acts to undercut the photoresist and acts to etch aluminum below the photoresist in "protected" areas. Furthermore, the use of the combination of carbon tetrachloride and chlorine as etchant gases in the plasma etching of aluminum acts to form a polymer film, which is not capable of being removed from the substrate surface.